Such devices have been known in the past. See German patent Specification No. 179,675, German Patent Specification No. 224,700, German Patent Specification No. 873,908. They are all based on the following principle:
In a shallow housing having a viewing window that is covered, if desired, by a transparent plate, there is a pile of pictures, the uppermost one of which is exposed to view through the viewing window and is supported at its edges. A pressure arrangement using a spring holds the pile pressed upwards against the window or the frame thereof. Laterally, the housing has an aperture through which a slider member may be pulled out; the withdrawal movement of the slider member is limited by stops. Near to the housing aperture there is arranged a member referred to here and hereinafter as a separator which covers this aperture transversely and at the top and the bottom leaves clear a narrow slot for a picture to pass through. As the slider member is pulled out, the uppermost picture is pulled by a hook-like transporter through the top slot, wherein the remainder of the pile is supported against the separator and during the return stroke of the slider member the picture is pushed through the bottom slot and under the pile again, or more accurately, between the pressure arrangement and the bottom-most picture of the pile. The second uppermost picture now lies in front of the viewing window and the procedure can be repeated as often as desired.
The spring pressure arrangement serves not only to position the uppermost picture against the viewing aperture; a comparatively low force would be sufficient for that. It must also, however, see that the hook-like transporter engages properly even when the pictures of the piles are deformed, spoiled and arched. The forces to be applied are therefore considerable.
This makes the change-over mechanism not only awkward but also results in a stress on the pictures which for photographs, or example, is inadmissible. The uppermost picture is clamped between the window or the window frame and the second uppermost picture and slides on both as the pressure arrangement is pulled out, and the picture is likewise subjected to considerable sliding friction when reinserted. Furthermore, the picture is crushed end on and thus compressed in the longitudinal direction which is troublesome, particularly during the reinsertion procedure, as the picture is not supported by the pile.